The invention relates to a FM receiver as described in the preamble of claim 1. The invention also relates to control means for use in such a FM receiver. The invention also relates to a method for controlling an IF.
FM receivers are known in the art. In the last decades, the number of FM transmitters worldwide has steadily increased. One consequence of this xe2x80x9csqueezingxe2x80x9d of more channels into the FM band has been an increased channel selectivity requirement for the (radio) FM receivers. To meet this requirement the filters in the FM receivers have to be made more selective.
An adaptive solution is to switch between two fixed filters with different bandwidths depending on some detection criterion. Switching between a number of fixed filters is quite costly, gives only discrete bandwidth solutions and still requires selected filters with respect to the respective center frequency.
A more elaborate solution is to incorporate channel filters with truly variable bandwidth. Such a system is known from the Philips integrated circuit TEA 6850 which uses integrated filters were the bandwidth is a function of the adjacent channel residue in the ultrasonic frequency range (this is above 100 kHz.).
Although this system is used in the market it has a number of limitations. The most important shortcoming of this system is that at low RF levels and large wanted frequency deviations, the channel filter goes into a so-called latch up mode with undesirable consequences. This is because, under the above signal conditions, high frequency distortion products are generated which are interpreted as adjacent channel signals. The control loop then reduces the IF filter bandwidth to counteract this apparent undesired signal, but in fact increases the unwanted distortion. This continues until the IF filter bandwidth is stuck in a minimum state.
It is an object of the invention to provide a FM receiver having a good dynamic selectivity and at the same time has a large frequency deviation tolerance.
A second object of the invention is to provide control means for use in a FM receiver.
A third object of the invention is to provide a method for controlling an IF filter.
An optimum instantaneous bandwidth can be defined by taking into account not only the adjacent channel but also the RF level and desired modulation as well.
Further not only the bandwidth can be controlled dynamically but also other attributes such as the decay response and allowability of break throughs.
A side advantage of this new bandwidth control is an improved FM receiver sensitivity.
A FM receiver of an embodiment can be used both for analog as well as digital implementation.
In accordance with an exemplary embodiment, an FM receiver comprises: an RF section; a tunable mixer stage that frequency converts a desired RF FM reception signal into an intermediate frequency (IF) signal; a controllable IF filter; an FM demodulator which demodulates the IF signal; and a control means to control a bandwidth of the controllable IF filter. The control means further comprises a first input that receives a first input signal in dependence of a received RF level signal; a second input that receives a second input signal in dependence of a demodulated signal; an output that is coupled to the controllable IF filter; and an offset detector that detects an offset in said demodulated signal.
Embodiments of the invention are described in the dependent claims.
By using both the dynamic selectivity means and the threshold extension means for obtaining a control signal to be supplied to the controllable IF filter the behavior of the FM receiver is further improved.